In recent years, thermoplastic elastomers, such as styrene, olefin, polyester, polyamide, and polyurethane thermoplastic elastomers, which are rubbery soft materials, require no vulcanization step, and have the same moldability as thermoplastic resins, have drawn attention from the viewpoints of the rationalization of the process, recycling and the like, and have been extensively used in fields, such as parts for automobiles, parts for domestic electric appliances, parts for medical equipment, electric wires, sundries, civil engineering sheets, building materials and the like. Among others, olefin thermoplastic elastomers comprising mixtures of crystalline propylene polymer resin with an olefin copolymer rubber, such as an ethylene/propylene copolymer rubber or an ethylene/propylene/non-conjugated diene copolymer rubber, are relatively inexpensive and hence have drawn attention as cost-effective materials.
This type of olefin thermoplastic elastomers, however, because they are in the form of a mixture, are likely to cause coarse dispersion or heterogeneous dispersion of the rubber. Therefore, as compared with other thermoplastic elastomers or conventional vulcanized rubber, they are inferior in balance between softness and tensile properties, such as tensile breaking strength and elongation, and balance between softness and low-temperature impact resistance, and, for example, involve a problem that, on substantially the same level of softness, they are inferior in tensile properties and low-temperature impact resistance. In addition, the above type of olefin thermoplastic elastomers have unsatisfactory heat resistance.
In order to solve these problems, processes for producing a composition comprising a crystalline propylene polymer resin and an ethylene/propylene copolymer rubber by polymerization have been proposed. For example, Japanese Patent Laid-Open Publication No. 2054139/1991 discloses an elastoplastic polypropylene composition, produced by successive polymerization, comprising: (A) 10 to 60 parts by weight of a crystalline homopolymer or copolymer of propylene; (B) 10 to 40 parts by weight of a crystalline propylene/ethylene copolymer component insoluble in xylene at room temperature; and (C) 30 to 60 parts by weight of a noncrystalline propylene/ethylene copolymer component soluble in xylene at room temperature and having an ethylene content of 40 to 70% by weight, and Japanese Patent Laid-Open Publication No. 25367/1994 discloses an elastoplastic polypropylene composition, produced by successive polymerization, comprising: (A) 10 to 50 parts by weight of a crystalline homopolymer or copolymer of propylene; (B) 5 to 20 parts by weight of a crystalline propylene/ethylene copolymer component insoluble in xylene at room temperature; and (C) 40 to 80 parts by weight of a noncrystalline propylene/ethylene copolymer component soluble in xylene at room temperature and having an ethylene content of less than 40% by weight.
According to studies conducted by the present inventors, however, although the former composition has excellent balance between softness and low-temperature impact resistance, the balance between softness and tensile properties is poor, while the latter composition has excellent balance between softness and tensile properties, but is poor in balance between softness and low-temperature impact resistance. That is, it has been found that, in both compositions, a simultaneous improvement in balance between softness and tensile properties and balance between softness and low-temperature impact resistance could have not been achieved, and, in addition, the heat resistance is still unsatisfactory.